1. Field of the Invention
The present invention relates generally to implantable cardiac devices and, more particularly, to an implantable cardiac device with the capability of switching between arrhythmia prevention modes.
2. Background Art
An implantable cardiac device is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable cardiac devices include, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device capable of delivering therapy to prevent or terminate a fast heart rate or a tachycardia. An ICD employs a battery to power its internal circuitry and to generate electrical therapy. The electrical therapy can include, for example, pacing pulses, cardioverting pulses and/or defibrillator pulses. This is in contrast to a “pacemaker” which is an implantable device specifically intended to treat slow heart rates or bradycardia.
An ICD also provides all the features of a pacemaker. However, a pacemaker does not provide all of the therapy that can be provided by an ICD. While the term “ICD” is used throughout the specification, it is to be understood that similar techniques as described herein can be applied in a pacemaker. A pacemaker cannot initiate therapy if a tachyarrhythmia occurs, but it can stabilize the ventricular rate in the setting of atrial fibrillation and can prevent atrial and ventricular arrhythmias.
There is indirect evidence that ventricular instability during an atrial arrhythmia can be a mechanism for inducing a ventricular arrhythmia. A recent study showed that the presence of atrial fibrillation is an independent predictor of appropriate ICD therapy, with ICD therapy being 1.8 times more frequent in patients in atrial fibrillation versus patients in sinus rhythm. More information regarding this study can be found in “Association Between Atrial Fibrillation and Appropriate Implantable Cardioverter Defibrillator Therapy: Results form a Prospective Study,” by Gronefeld, G, et al., Journal of Cardiovascular Electrophysiology, 11(11), pp. 1208-1214 (2000).
It has also been shown that overdrive ventricular pacing can reduce ventricular extrasystoles and arrhythmias. (See Pekarsky, V., et al., “Prevention Of Recurrent Life-Threatening Ventricular Arrythmias By Temporary Cardiac Pacing,” Acta Med Scand, Vol. 21, pp. 95-99 (1985).) In addition, it has been shown that pacing the ventricle slightly over the mean intrinsic ventricular rate during atrial fibrillation can significantly reduce the number of premature conducted beats (PCBs) from the atria. (See Wittkampf, F. and DeJongste, M., “Rate Stabilization By Right Ventricular Pacing in Patients with Atrial Fibrillation,” PACE, Vol. 9(Part II), pp. 1147-1153 (1986).)
What is needed is a device, such as an ICD, that takes advantage of such evidence to reduce the occurrence of arrhythmias by delivery of appropriate electrical therapy.